1. Field of the Invention
This invention relates to linear actuators and more particularly, to a novel linear actuator apparatus and method wherein a plurality of rods fabricated from a magnetostrictive material are used both as the motive force and the clamping forces for providing incremental movement between a steel drive rod and the linear actuator apparatus.
2. The Prior Art
A linear actuator, as the name implies, refers to a member of that family of actuators that move in a longitudinal or linear manner as distinguished from rotary actuators such as electric motors, reciprocatory engines, and the like, that operate on the principle of rotary motion. A hydraulic piston, for example, is one type of linear actuator and involves the flow of hydraulic fluid under pressure to move the piston in the appropriate direction as controlled by the valving sequence of the hydraulic fluid. Another type of linear actuator is the solenoid wherein the axial movement of a centrally disposed, spring-biased shaft is controlled by electromagnetic forces created with an electromagnetic coil surrounding the shaft. The solenoid is generally a simple on/off type device and is not well suited for fine adjustments of the shaft throughout its travel distance. Other linear actuators involve an axial rod having a worm gear system for translating the rotary motion of an electric motor, for example, into linear movement of the rod.
Another type of motive force in addition to the foregoing hydraulic and electromagnetic forces is that of a relatively new material known as a magnetostrictive material. As the name implies a magnetostrictive material changes length under the influence of an electromagnetic field. Actuator-type devices which take advantage of this change of length characteristic of magnetostrictive materials have been disclosed and include a hydraulic pump proposed by Cusack (U.S. Pat. Nos. 4,726,741; 4,795,317; 4,795,318; 4,804,314; and 4,815,946). In this device the piston for the hydraulic pump is constructed of a magnetostrictive material that is caused to extend under the magnetic influence of an electromagnetic field. In one preferred embodiment, the cylinder is constructed of a negative magnetostrictive material which decreases in length in the presence of the magnetic field.
Glomb, Jr. (U.S. Pat. No. 4,766,357) discloses a linear magnetostrictive actuator. This device includes demagnetization compensation having improved displacement accuracy.
Engdahl et al. (U.S. Pat. No. 4,927,334) disclose a liquid pump driven by rods of magnetostrictive material. The rods are arranged in pairs and surrounded by coils supplied with electrical current. The first rod and second rod cooperate in moving a piston in a cylinder to create the pumping action.
Teter et al. (U.S. Pat. No. 5,039,894) disclose a linear motor wherein the actuator rod of the linear motor is made of a magnetostrictive material.
Dixon (U.S. Pat. No. 5,101,183) discloses a clamp made of magnetostrictive materials. The clamp is designed to prevent electrical power conductors from being forced apart by the magnetic fields produced when high currents flow in the conductors. One proposed application is that involving the conductors in a railgun.
In each of these references, the driving element is made of a magnetostrictive material. However, magnetostrictive materials are not only expensive but also highly susceptible to wear if used as the driving element. It would, therefore, be an advancement in the art to provide a linear actuator wherein the force transmitting rod is fabricated from steel or some other suitable metal and where the motive elements of magnetostrictive material are confined as relatively small elements. It would also be an advancement in the art to provide a linear actuator wherein the magnetostrictive elements are configured such as to be protected against wear. Such a novel apparatus and method is disclosed and claimed herein.